Internal combustion engines usually have cylinder valves which are opened and closed by a rotary camshaft. The camshaft has cams spaced along the shaft. Associated with each cam is a cam follower attached to a rocker which engages a valve that is biased to its closed position by an appropriate spring. When the camshaft is rotated, the cam lobes rock their respective rockers causing the valves to open and close in sequence. The angular offsets and shapes of the cam lobes determine the valve lift, opening duration and timing. Usually, the axial position of the camshaft is fixed so that the valve motion profiles remain constant as the camshaft rotates.
Internal combustion engineers have known for some time that engine performance can be improved by adjusting the valve motion depending upon engine speed and load. To this end, some engines have been designed with axially shiftable camshafts. Such camshafts have cams whose rocker-actuating lobes have profiles which change depending upon the axial position of the shaft. For example, each lobe may have a complex surface shape which varies along the axis of the shaft. Thus, by positioning the camshaft in a desired axial location, the valve lift, valve opening duration and valve phase tinning may be set according to the particular requirements of the engine. Engines such as this are disclosed, for example, in U.S. Pat. Nos. 3,618,573 and 5,211,143.
FIGS. 1A and 1B illustrate the valve train of a typical adjustable valve system according to the prior art. The system incorporates a camshaft 2 with one cam 3, one pivoted cam follower 4, and one rocker 5 for each engine valve 6 that is biased to the closed position by a spring 7. The rocker is pivotally supported by a connection 8 to the engine cylinder head 9 so that follower 4 is in contact with the associated cam 3 so that when the camshaft 2 is rotated, the rockers 5 swing up and down thereby opening and dosing the valve 6. By shifting the axial position of the shaft 2, the valve lift, valve opening duration and valve phase timing may be set according to the particular requirements of the engine.
The prior systems of this type are disadvantaged in that to avoid mechanical collision of parts, the camshaft is limited to a relatively short allowable travel length T.sub.p (FIG. 1A). This necessitates the use of a relatively steep cam slope S.sub.p in order to accommodate the desired change in valve lift over that available travel length T.sub.p. The travel length may be increased marginally in some prior systems by reducing the width W.sub.p of cam follower 4, but this results in increased contact pressure on and wear of the cam and cam follower. Short shaft travel length T.sub.p, steep cam slope S.sub.p and narrow follower width W.sub.p produce several disadvantages.
First, the steeper axial slope of the cam 3 induces larger oscillating axial forces into the system which make unwavering axial positioning of the camshaft more difficult to accomplish, necessitating the use of an expensive camshaft axial positioning mechanism.
Further, since the camshaft can only move axially a short distance, that short axial travel of the shaft has to produce a large increase in engine power. Consequently, high precision tolerances are required to prevent significant changes in engine power from occurring due to axial play in the system. This precision requirement necessitates the use of an even more expensive camshaft axial positioning mechanism.
Additionally, with short axial travel length cams, the cam surface curvature must be more broadly rounded in order to avoid double contacts and impacts between the cams and their followers. In systems designed to achieve both continuous line contacts by the cam followers and no impacts between the cams and cam followers, as camshaft travel length is reduced, cam lobe variability and, therefore, valve motion adjustability is lessened. This constraint on valve motion adjustability has limited the utility of many prior art systems. Systems of the type described in the above two patents which utilize tappet-type cam followers are most limited in terms of valve motion adjustability.
The machine design problems just described can be lessened by limiting the range of variability of the adjustable valve system. However, this compromises the utility of the system. The limitations of prior art adjustable valve systems are recognized in the publication "A Survey Of Variable-Valve Actuation Technology", by T. Ahmad, Society of Automotive Engineering, Paper No. 891674, 1989 and "Type Synthesis Of Mechanisms For Variable Valve Actuation", by Charles W. Wampler, Society of Automotive Engineering, Paper No. 930818, 1993. The former article specifically states that in a survey of existing adjustable valve mechanisms, three-dimensional cams provide too narrow a range of duration and lift to allow adequate engine load control.
Still another disadvantage of prior art adjustable valve systems of the type described in the above two patents is that the valve rockers include flanges which transmit lateral forces from the camshaft to the valve stem ends. These lateral forces on the valve stem ends can cause accelerated wear of the valves and valve stem guides, as well as oil and/or fuel leakage past the valve stem guides, vibration and, in some cases, even breakage of the valve stems.
Additional disadvantages of the prior art adjustable valve systems include machine design complexity, excessive parts count and excessive manufacturing cost.